Vibrations in aerospace structures create many important and difficult engineering problems. In certain aircraft and rotorcraft applications, structural vibrations may increase interior cabin noise levels and/or accelerate material fatigue. Identifying the sources of troublesome vibrations and subsequently developing strategies for reducing these vibrations has been and continues to be the focus of a large body of research.
Interior noise levels in certain propeller driven aircraft, rotorcraft, and the more advanced high-speed turboprop aircraft are, in general, higher than desirable. In these vehicles, noise is generated from both airborne and structure-borne sources. Airborne noise arises from acoustic sources such as the interaction of the propeller wake/vortex with the aircraft fuselage or the impingement of jet exhaust directed at the fuselage. Structure-borne noise is a result of vibrations from the engine or vibrations from the interaction of the propeller wake/vortex with the wing surface being transmitted via the aircraft structure to the main cabin. In addition, the flexible attachment of the rotor blades to the rotor hub and gear meshing in the main rotor gearbox of rotorcraft may also generate extremely high cabin noise levels. Thus with many of the sources of interior noise identified, the problem becomes that of reducing the resulting noise vibration levels within the aircraft or rotorcraft cabin.
Structural acoustic control is a method for reducing the interior cabin acoustic field by reducing vibrations due to external excitation sources before they prop a gate to and excite the coupled interior structural acoustic modes of the aircraft fuselage. Direct airborne induced disturbances may be inhibited from propagating to the fuselage by altering the stiffness of the wing and fuselage, by adding surface damping treatments to the wing and fuselage, by adding blocking masses to the aircraft structure, by using passive and active vibration absorbers, by using resistively shunted and resonantly shunted piezoceramics, or by using active vibration control. Vibrations that propagate from the engine may be reduced by passive and active isolation, by active control, and by passive and active vibration absorbers.
Of particular interest to the present invention is the use of vibration control devices for actuator and/or absorber applications. Vibration control devices are conceptually simple devices consisting of a mass attached to a structure via a complex spring. When used as an actuator, the vibration control device produces vibration at a predetermined frequency in an attached structure. When used as an absorber, the vibration control device reduces vibration in an attached structure caused by a disturbance.
Vibration absorbers are typically used to minimize vibration at a specific frequency often associated with a lightly damped structural mode. For the device to operate at the correct frequency, the mass and stiffness must be chosen correctly so as to tune the device to the frequency of the offending mode or disturbance. The fact that a vibration control device may only be used at a specific frequency, however, can sometimes be the largest drawback of using these devices.
Passive vibration absorbers (PVAS) have been used in the aviation industry for quite some time. For example, the DC-9 uses a set of four PVAs attached to each engine pylon to reduce the aft cabin noise associated with the operating spool frequency of the engines. Similarly, both the Fokker F27 and the Saab 340 aircraft use PVAs attached directly to the fuselage frame to lower interior cabin noise levels. In these applications, the absorbers provide adequate vibration attenuation at specific frequencies. Performance can be seriously degraded, however, if the disturbance source changes frequency. If this occurs, the devices must be physically re-tuned. Re-tuning the vibration control devices may often be either impractical or impossible, hence there is a need for a vibration control device with properties that are easy to alter.
An object of the present invention is to provide a novel and improved vibration control device having a piezoceramic element with shunt capacitor.
Another object of the present invention is to provide a novel and improved vibration control device that is tunable to a desired frequency.
Still another object is to provide a control system for tuning a piezoceramic vibration control device to a changing disturbance frequency.
Yet another object of the present invention is to change stiffness of the piezo element by means of a shunt capacitor.